As you expand, you'll inevitably conquer provinces with different cultures and religions. Your Steward can 'Promote Culture' in a county, slowly converting it to your primary culture, which reduces opinion penalties. Your Court Chaplain/Imam/Rabbi can 'Convert Faith' in a county, which is vital for reducing unrest and unlocking Holy War CBs. Prioritize converting your core territories and those with high development potential. Cultural and religious unity strengthens your realm.
